Method of introducing additives in steelmaking

ABSTRACT

The additives, in particulate solid form, are conveyed pneumatically in a divergent stream from a pneumatic gun to impinge upon the molten iron and mix therewith. The gun is spaced above a surface of molten iron such that the pneumatically conveyed stream including the additive has a central axis which is either horizontal (or at an acute angle to the horizontal). The gun is prefereably adjustable for adjustment of the stream angle, and the stream may be either added to pouring metal or to cover a surface of the molten iron.

The present invention relates to a method of introducing additivesduring steelmaking, either to molten iron in a ladle or the like, or tomolten iron while the latter is being poured.

Conditioning of steel at various stages in steelmaking processes oftenrequires the introduction at relevant process stages of variousadditives (such additives are often known as conditioning agents becausethey “condition”, or change the properties and/or the composition of,the resulting steel). In conventional arrangements, such additives maybe introduced by gravity feed (by flow of the additive from a hopper orthe like placed above the molten metal), or by direct injection intomolten metal or slag, using, for example, a lance arranged verticallyabove the hot metal (the latter being typically in a runner fordirecting molten pig iron tapped from a blast furnace into a hot metalladle).

U.S. Pat. No. 4,601,749 discloses a method of the latter type, in whicha lance is arranged vertically above such a hot metal runner. The methoddisclosed is relatively inflexible in its operation, and requires theinjection lance to be arranged in the very aggressive environment ofjust above the surface of the molten metal in the hot metal runner. Animproved arrangement has now been devised.

According to a first aspect of the present invention, there is provideda method of introducing at least one additive into molten iron in asteelmaking process, in which the additive in particulate solid form isconveyed pneumatically to impinge upon the molten iron and mixtherewith, the additive being pneumatically conveyed in a divergentstream from a pneumatic conveying outlet (or gun) spaced above a surfaceof molten iron present in a receptacle (such as a ladle), the conveyingoutlet being such that the pneumatically conveyed stream including theadditive has a central axis which is either horizontal or at an acuteangle to the horizontal.

It is preferred that the axis is adjustable from a first angle to asecond angle inclined to the horizontal. Such an adjustable outletenables the pneumatically conveyed stream to be accurately targeted to,for example, impinge upon a pouring stream or to substantially cover asurface of molten iron in a receptacle.

When the pneumatically conveyed stream is to substantially cover asurface of molten iron in a receptacle, the outlet is above, andpreferably outwardly spaced from an outer edge of the receptacle. Theterm “iron” as used herein encompasses any predominantly ferrous metalor alloy (which may contain incidental ingredients or impurities)suitable for use in a steelmaking process. It specifically includes thematerial being poured from a converter vessel in the course of asteelmaking process.

The use of a pneumatically conveyed stream provides several benefits,including lower cost, and enhanced dispersal of the additive in themolten iron. In terms of cost, there is no requirement for a speciallydesigned treatment station, because the relevant outlet nozzles (“guns”)can be readily added to an existing plant structure, and an expensiveand short-lived lance is not needed.

The central axis of the stream is one about which the stream diverges,to form a substantially divergent conical stream of pneumaticallyconveyed particulate additive, which impinges upon the molten iron inthe form of projectiles.

The first angle may be substantially horizontal or at an acute angle tothe horizontal; it should not be vertical.

It is particularly preferred in the method according to the inventionthat the pneumatic conveying outlet can be adjusted such that the angleof the axis of the stream can be optimised, depending on the applicationand the location of the surface of the molten iron.

When the pneumatically conveyed stream which includes the additive has acentral axis which is substantially horizontal, the additive ispreferably added to flowing molten iron during pouring of the latter(typically during pouring into a ladle or the like, the latter thereforeincluding the surface of the molten iron referred to above). In thisembodiment, the kinetic energy of the poured iron can assist in thedispersion of the additive directed thereto in a pneumatically conveyedstream. Such a method in which the additive is directed to molten ironbeing poured into a ladle or the like is described in more detail below.

When the central axis is at an acute angle to the horizontal, theadditive may be added to the flowing molten iron during pouring, or (ina preferred embodiment of the invention) the additive may be directedtowards the surface of molten iron in the receptacle.

When the stream including the additive is directed towards the surfaceof the molten iron in the receptacle, the additive is preferablyconveyed to reach below the aforesaid surface, penetrating through slagor other surface covering thereon. It is particularly preferred in thisembodiment of the invention that the stream is directed so as tosubstantially cover the entire surface of the molten iron in thereceptacle, and impinge at least in part on sidewalls of the receptacle.This is contrary to the teachings of the abovementioned U.S. Pat. No.4,601,749, where the added stream is directed vertically downwards tothe surface of the molten iron with very little divergence of thestream.

According to the invention, however, the “footprint” of the conveyedadditive preferably covers the entire surface of the molten iron in thereceptacle. This can ensure, for example, that the total surface ofmolten iron in a ladle may be covered without the requirement tophysically move either the conveying outlet or the conveyed stream so asto scan the entire molten iron surface. It is, however, possible toarrange for the stream to scan the surface, or to provide a plurality ofsuch conveying outlets.

In further embodiments, different nozzles can be used for differentapplications, so that a widely divergent stream can be provided in someembodiments and a stream with little divergence can be formed in othercircumstances.

In most applications of the present invention, it is preferred that theconveying gas will be air, although inert conveying gasses (such asnitrogen) may be preferred in some instances.

The additive may be in any suitable particulate form, such as tablets,pellets, briquettes or powder. The density and composition of suchtablets, pellets, briquettes and the like may be tailored in order topenetrate to predetermined depths in the molten iron at a predeterminedrate. This enables the additive to be tailored to perform specificreaction requirements at specific depths and times. For example, thespecific density and composition of tablets introduced into molten ironmay be selected to break down quickly when in the presence of hot slag,but to react with the specific chemical components in the molten ironwhich are targeted for neutralisation or alteration.

The predetermined specific density of the particulate additive canensure that the particles penetrate into, and remain in, the slag(rather than descending into the liquid iron below) but resist flaringoff on the surface.

Significant upward thermal currents exist above the surface of molteniron, which would hinder the deployment of additive by gravity feed. Theuse of the conveying gas delivery arrangement in the method according tothe invention can ensure that the effect of the upward thermal aircurrents above the molten iron can be compensated for.

The delivery pressure and velocity of the conveying gas can therefore betailored, depending upon the ‘sinkage’ requirements of the additivebeing delivered and the upward thermal currents encountered above themolten iron in the relevant process stage. Typically, the dispensingpressure of the conveying gas will preferably be substantially in therange of 7 bars plus or minus 20%. The discharge rate of dispensedmaterial is preferably substantially in the range 0.5 to 15 m³ per hour.

Preferably, the conveying outlet comprises a nozzle, preferably adiverging nozzle arranged to induce a diverging outlet stream which fansor diverges outwardly in a direction away from the nozzle.

In some embodiments, the molten iron is preferably contained in asubstantially molten state in a receptacle, such as a ladle, flowpathchannel, duct or the like. It is preferred that the receptacle is in theform of a ladle, and that (in this embodiment) the conveyed stream isarranged to impinge walls of the ladle substantially surrounding thesurface of the molten iron therein.

In other embodiments, it is preferred that the surface of the molteniron should be below a flow of metal being poured thereinto; in thisembodiment, it is preferred that the additive is pneumatically conveyedaccording to the invention into the pouring stream of metal. Thisenables the available kinetic energy of the flowing stream of metal tobe efficiently utilised to aid dispersion of the

Additive, without the need to use expensive gases for stirring.Furthermore, as the addition in this embodiment takes place during anexisting process (that is, the usual pouring from one ladle to another),no additional process step or time is needed. The additive can inaddition be dispersed intimately throughout the molten iron so that theadditive is able to react in a manner of optimum efficiency.

In some embodiments of the invention, it is preferred that the additivecomprises a multiplicity of shaped elements (such as tablets, briquettesor the like), which preferably include aluminium when the additive is tobe used for reheating steel during secondary steel making, or for“killing” slag on the surface of a steel ladle. Such shaped elementspreferably comprise compressed divided material, which form individualself-supporting elements.

Especially when such elements are used for killing slags, it may bebeneficial to include calcium carbonate, such that when reaction takesplace with the slag, carbon dioxide will be released, which will thengently bubble and effectively stir in the aluminium.

It is sometimes preferred that shaped elements such as those describedabove should include swarf, chippings, grindings or other dividedaluminium, compressed to form self-supporting shaped elements; they mayoptionally contain iron (typically in form of an oxide, which isespecially preferred to be in the form of millscale, because the latterclosely mirrors the specifications generally required by a steelmanufacturer).

The shaped elements may additionally or alternatively include one ormore non-aluminium materials, preferably arranged to have a conditioninginfluence upon molten iron or slag. For example, the shaped elements mayinclude slag conditioning additives and/or ladle insulating powders.

One or more of the following materials may be included in the additiveused according to the invention, depending upon user requirements: lime,magnesia, alumina, fluorspar, silicon or the like. Each of thesematerials is commonly used in steelmaking processes, generally in orderto aid process control.

Such additives may be bound in the shaped elements as divided material(fine or coarse); in certain embodiments they may be distributedthroughout a shaped body predominantly of aluminium.

In one embodiment of the invention, the additive may include or consistessentially of lime, which may be in the form of relatively smallbriquettes. In this embodiment, the lime is typically pneumaticallyconveyed or gunned into a tapping stream or the like in which the ironis tapped from a converter vessel and the lime is added or gunned intothe stream in a tight cone. This can reduce dust in adding the lime andcan avoid large amounts of the lime remaining unreactive on the surfaceof a ladle or the like.

In a further embodiment of the invention, the additive may be in theform of small briquettes containing lime, aluminium and soda ash, whichcan thereby be used as a desulphurising medium for molten iron. In thisembodiment, the additive can be fired into a poured stream of the metal(for example when the latter is being poured from a blast furnacetorpedo car into a BOS plant transfer ladle. The pouring action releaseslarge amounts of kinetic energy and additive material can be drawn andstirred into the molten iron without the costs and delays associatedwith conventional systems.

According to a second aspect, the invention provides steelmakingapparatus comprising:

i) a receptacle, channel or flowpath containing molten iron;

ii) a pneumatic conveying outlet spaced above the surface of the molteniron, the conveying outlet being arranged to deliver additive in apneumatically conveyed divergent stream to penetrate into the molteniron,the conveying outlet being adjustable so that the pneumaticallyconveyed stream can have either a central axis which is substantiallyhorizontal, or a central axis which is at an acute angle to thehorizontal.

The invention will now be further described in specific embodiments, byway of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic end view showing certain features of an exemplaryembodiment of a method according to the invention;

FIG. 2 is a schematic side view showing in more detail features of themethod illustrated in FIG. 1;

FIG. 2a shows in more detail the connection of the gun shown in FIGS. 1and 2 to the wall of a converter housing;

FIG. 3 is a schematic view of an alternative embodiment in which lime isgunned into a pouring stream; and

FIG. 4 is a schematic view of an embodiment similar to FIG. 3, for thepurpose of desulphurising of molten iron being transferred to a ladle.

Referring to FIGS. 1 and 2, a ladle 1 containing molten iron and slag ispositioned below a nozzle outlet of a gun 2. Gun 2 is connected via apneumatic line 3 (not shown in FIG. 1) for distributing additivesupplied from a hopper (not shown) to the surface 5 (see FIG. 1) ofmetal in the ladle 1. The metal stream from outlet 2 has diverging edges4,4′ and a central axis 6 which is inclined to the horizontal (as seenmore clearly in FIG. 2).

The gun 2 is pivotally mounted at 7 to the wall 8 of a converter housing9; the pivotal mounting is such that the gun can pivot about two axes topermit spraying accuracy. More details of the pivotal mounting are shownin FIG. 2a; it can be seen that the pivotal mounting for the gun 2 isclamped to the lower edge of an access hatch 10 cut into converterhousing 9, the hatch having a deflection hood 11.

The mounting allows the nozzle outlet of the gun to move both up anddown, and left and right. A clamp 12 secures the nozzle outlet of gun 2to the pivotal mounting 13 and can be slackened and the gun withdrawnfor quick changeover.

A hopper store (not shown) delivers particulate additive material intablet/pellet form (or the like) to line 3 and then to gun 4 to bedistributed over the surface 5 of the molten metal (and slag) in ladle1.

The pneumatic conveying system typically has a range of output dischargerates, typically in the range 0.75 to 10 m³ per hour, the desired outputbeing tailored to the process condition required for a particularapplication and the volume and density of the additive material beingconveyed.

The process parameters to which the output needs to be tailored are:

i) tablet/pellet size and/or density for the additive;

ii) thermal updraft from the molten iron in the ladle 1; and/or

iii) desired penetration depth (and/or rate of penetration) into themolten iron in ladle 1.

The pneumatic gun 2 is tailored such that the height of its nozzle abovethe ladle 1 can ensure that the divergent edges 4,4′ of the spray of theconveying gas and additive are dimensioned to substantially cover thewidth dimension (or span) of the surface 5 across ladle l, as shown inFIGS. 1 and 2. This ensures that there is no need for scanning of theoutput spray.

Utilising the pneumatically conveyed additive ensures rapid uniformcoverage of the relevant additive over the surface 5 of the molten ironin the ladle 1. Additionally, the pressure of the conveying gas may betailored to ensure that thermal updraft from the molten iron iscompensated for, permitting additive to be introduced to penetrate torequired depths within the molten iron at specific rates to perform aspecific chemical interaction within the molten iron. The additive maybe aluminium, aluminium based or other material such as(non-exhaustively) lime, magnesia, alumina, fluorspar, millscale, steelturnings or the like. Each of these materials is commonly used insteelmaking processes in order to aid process control and steelconditioning.

Typically, the additive is compressed (or otherwise bonded) from nonself-supporting agglomerations of relevant material into the form ofpellets, tablets, briquettes or the like. Such briquettes may includeone or more combinations of the additive in varying proportionsdepending on application requirements.

The density of the relevant tablets, pellets, briquettes or the like ispre-selected to meet the required performance characteristics. Forexample, shaped bodies formed by briquetting for use according to theinvention may have a density in the range 2.2 to 2.8 Kgm⁻³; whereasshaped bodies formed by tableting or pelletizing may have a density inthe range 1.4 to 4 Kgm⁻³.

Referring now to FIG. 3 (in which like parts are denoted by likereference numerals), there is shown a schematic view of an alternativeembodiment, in which a converter vessel 13 is arranged to pour molteniron in the form of a stream 14. While in flight, the molten iron isimpinged by a further stream 15 of lime, directed from pneumatic gun 2.

The small lime briquettes are fired into the poured stream of molteniron during tapping. The briquettes are dragged down into the ladle andmix with the molten metal where the lime can mix efficiently.

Referring now to FIG. 4 (in which again like parts are denoted by likereference numerals), the iron is being poured from a mixer 20 to a ladle1; while in flight, the molten iron stream 14 is impinged by a asubstantially horizontal diverging stream 15 of desulphurising pelletsfrom gun 2.

What is claimed is:
 1. A method for killing slag on the surface ofmolten steel present in a receptacle, in which additive materialincluding aluminum material in particulate or divided solid form is gasconveyed to the slag in a conveying gas in a divergent stream fromconveying apparatus spaced above the slag surface, the conveying gaspressure being tailored with respect to the conveyed additive materialto cause the conveyed additive material to penetrate into andsubstantially remain in the slag.
 2. A method according to claim 1,wherein gas conveyed additive is directed to impinge with slag extendingover the majority of the surface area of the molten steel in receptacle.3. A method according to claim 1, wherein the additive material is boundinto a multiplicity of shaped elements.
 4. A method according to claim3, wherein the shaped elements comprise compressed divided materialforming individual self-supporting elements.
 5. A method according toclaim 4, wherein the shaped elements are selected from the groupconsisting of swarf, chippings, and grindings.
 6. A method according toclaim 1, wherein the outlet of the conveying apparatus is above andoutwardly spaced from an outer edge of the receptacle.
 7. A methodaccording to claim 1, wherein the density and/or composition of theadditive material is/are tailored with respect to parameters of theconveying gas so as to provide that the additive material penetratesinto the slag to a predetermined extent.
 8. A method according to claim1, wherein the gas conveyed stream is configured to ensure that at leastthe majority of the surface of the slag in the vessel is covered by thegas conveyed additive.
 9. A method according to claim 1 wherein theadditive is gas conveyed at a dispensing pressure of conveying gassubstantially in the range 7 bars plus or minus 20%.
 10. A methodaccording to claim 1, wherein the additive is conveyed at a ratesubstantially in the range 0.5 to 15 m³ per hour.
 11. A method forkilling slag on the surface of molten steel present in a receptacle, inwhich additive material including aluminium material in particulate ordivided solid form is gas conveyed to the slag in a conveying gas in aninclined divergent stream from an inclined conveying apparatus spacedabove the slag surface, the conveying gas surface being tailored withrespect to the conveyed additive material to cause the conveyed additivematerial to penetrate into and substantially remain in the slag.